Have you Googled the term? Apart from the first page or so being dominated by that very recent event of the crowd intimidating people and many other people conflating that event with that slogan, you'll find plenty of articles about what it means: that choosing to not speak out about an issue helps support the status quo. In fact, I've generally seen it used to try to persuade people who don't want to support the status quo that staying quiet or trying to "not be political" is in fact supporting the status quo.
